It can be appreciated that direct-view Liquid Crystal Displays (LCDs) have been in use for years. These direct-view LCDs are widely used in laptop computers, desktop monitors, TVs, cellular phones, and other applications. Nearly all of the direct-view LCDs are of the nematic type (TN, VA, and IPS) and therefore require polarizers to observe the optical effects produced by the application of electric fields to the liquid crystal material. An example of a direct-view LCD is schematically shown in FIG. 1. Typically these direct-view LCD assemblies contain a backlight 100 that emits unpolarized light 102, a first polarizer 110, that transmits one plane of polarization 112, a planar array of individually modulatable liquid crystal elements 118, and a second polarizer 120. The function of the first polarizer is to transmit only one plane of polarization to illuminate the planar Liquid Crystal array 118. In all current designs, the polarizers 118 and 120 are both of the dichroic type (absorption-type) wherein one plane of polarization is transmitted while the orthogonal plane of polarization is absorbed.
One significant problem with this use of tandem absorption-type polarizers is very low net transmission of the backlight illumination. With absorption-type polarizers, theoretically at best, only 50% of the output of the backlight could be available to illuminate the LCD and viewed by the viewer 130. In practice, the efficiency of the absorption-type polarizers is less than perfect and only 40-45% of the available backlight could be transmitted through any one polarizer. Thus, the net transmission of the backlight illumination by the tandem pair of polarizers 118 and 120 is at best only 36-40%.
To make a higher fraction of the output of the backlight available to the liquid crystal layer and the viewer, several polarization recovery techniques have been developed over the years. These techniques convert some of the plane of polarization that would normally be absorbed and hence unavailable, into the plane of polarization that is used to illuminate the LCD. One representative example of a polarization recovery technique is shown in FIG. 2. The example shown in FIG. 2 is from U.S. Pat. No. 5,422,756. Other examples of polarization recovery can be found in U.S. Pat. Nos. 5,587,816, 5,751,388, and 5,973,833. The principle employed in the prior art of polarization recovery can be understood from FIG. 2. As before in FIG. 1, unpolarized light 102 is provided by backlight 100. If backlight 100 does not include diffuse reflecting properties, an additional diffuser layer 240 needs to be included. The diffusing properties are exploited to randomize the plane of polarization of the light 242; converting some of the p-polarized light into s-polarization and some of the s-polarized light into p-polarization. A reflecting polarizer 250 is fundamentally different than the traditional dichroic polarizers; it transmits one plane of polarization 252 and reflects the orthogonal plane of polarization 254. Polarization recovery is accomplished by reflecting the undesired reflected plane of polarization 252 and converting a fraction of it into the desired plane of polarization whereupon it is transmitted by the reflective polarizer 250.
FIG. 2 also illustrates a problem with such reflecting polarizers. The extinction ratio of reflective polarizer 250 is so poor that a second “clean-up” polarizer 110 is required for the display to produce adequate contrast; it doesn't eliminate the need for a backside absorption-type polarizer. Thus in all current direct-view LCD designs, even those employing the currently available polarization recovery methods, two absorption-type polarizers are used in tandem.
Prior art polarization recovery techniques using a reflecting polarizer are illustrated in FIGS. 3A-3B. Examples of the design of such prior art reflective polarizers may be found, e.g., in U.S. Pat. No. 5,422,756. The reflective polarizer 300 of FIG. 3A includes transparent substrates 312 and 314 fabricated with prismatic surfaces 316. Prior to joining the prismatic surfaces of 312 and 314, a multilayer dielectric coating is deposited on one or both prismatic surfaces. After the multilayer deposition the prismatic surfaces are bonded together without any voids that would impair their optical properties. This is a complex and expensive manufacturing process. As shown in close-up in FIG. 3B, the prismatic surface 316 may be made up of hundreds of layers of alternating polymer films arranged in a stack in which amorphous films 320 alternate with birefringent films 322. The stack of films transmits a p polarization 318-p while reflecting an s polarization 318-s of incident light. In addition to the problem described above there are additional problems with these reflective polarizers that are related to both performance and cost. Specifically retroreflective polarizers of the type illustrated in FIG. 3A and FIG. 3B use a complex multilayer structure that is expensive to manufacture. Thus, while prior art reflective polarizers do address the need for improved brightness in LCDs, the above problems and other known problems remain.
An alternative to using two absorption-type polarizers would be to use wire grid polarizers. A wire grid polarizer typically comprises an array of closely-spaced parallel conductive lines supported by a transmitting substrate. A perspective schematic view of such a polarizer is illustrated in FIG. 4. As can be seen, the polarizer 410 comprises an array of parallel conductive lines 412 on a transparent substrate 414. Each of the conductive lines is characterized by a thickness t, a width w and a periodic separation (or period) Λ with respect to the adjacent line(s). In operation, unpolarized light 416 is incident at an angle φ. (Note: the angle of incidence φ may be zero; that is the light 416 may be normal to the surface of the polarizer 410). A portion 418 of the incident light 416 is reflected while another portion 420 is transmitted. The reflected portion 418 is almost entirely s-polarized (electric vector parallel to the direction of the conductive lines 412) while the transmitted portion 420 is almost entirely p-polarized (electric vector perpendicular to the direction of the conductive lines 412).
Ideally, a wire grid polarizer functions as a perfect mirror for one plane of polarization (e.g. s-polarized light) and is perfectly transparent to the orthogonal plane of polarization (e.g. p-polarized light). In practice, even the most reflective metals absorb some fraction and reflect only 80 to 95 percent of incident light. Similarly, due to surface reflections, a nominally transparent substrate does not transmit 100 percent of incident light. Polarizer performance over the range of wavelengths and incidence angles of interest is characterized by the contrast ratios of the transmitted (Tp/TS) and reflected (RS/Rp) beams and optical efficiency (percentage of incident unpolarized light transmitted).
The overall behavior of a wire grid polarizer is determined by the relationship between (1) the center-to-center spacing, or periodicity, of the parallel conductive lines and (2) the wavelength of incident radiation. Only when the periodicity, Λ, of the lines is smaller than the wavelength of interest can the array behave like a polarizer. If the periodicity of the lines should exceed the wavelength of interest, the grid will function as a diffraction grating. Further, there exists a transition region, in which periodicity of the conductive lines falls in the range of roughly one-third to twice the wavelength of interest (i.e., λ/3<Λ<2λ). Large, abrupt changes are observed to occur in such transition region, namely increases in reflectivity coupled with corresponding decreases in reflectivity for p-polarized light. Such “Raleigh resonances” occur at one or more specific wavelengths for any given angle of incidence. As a result, wire grids having periodicities that fall within such transition region are unsuitable for use as wide band polarizers.
Wire grid polarizer technology offers some inherent advantages over dichroic absorptive polarizers. Wire grid polarizers operate by the reflection and transmission of light, and are therefore neither temperature sensitive nor does it absorb excessive amounts of energy. A dichroic absorptive polarizer, by contrast, operates by the selective absorption and transmission of light. As such, a dichroic based polarizer exhibits temperature sensitivity due to (a) sensitivity of the organic dye to degradation in the presence of heating and (b) thermal rearrangement (relaxation) of the polymer alignment achieved by stretching the polymer to line up the dye molecules. Such temperature sensitivity limits the types of manufacturing process that may be employed to create dichroic adsorptive polarizers. The relatively low temperature processes available are often sub-optimal in terms of yield, quality and cost.
Wire grid polarizers were developed for use in the millimeter-wave and microwave frequency ranges. They were initially unavailable for use in the infrared and visible wavelength ranges due to the inability of then-existing processing technologies (e.g. stretching thin wires over a mandrel) to produce parallel conducting lines of sufficiently small periodicity. The application of photolithography overcame the problem of attaining the requisite small periodicities. See, for example, U.S. Pat. No. 4,049,944 of Garvin et al. Covering “Process for Fabricating Small Geometry Semiconductive Devices Including Integrated Components” which teaches, in part, a method for fabrication of wire grid polarizers employing holographic exposure of photolithographic materials. Other applications of photolithography in methods for forming wire grid polarizers are taught, for example, in the following U.S. patents: U.S. Pat. No. 6,122,103 of Perkins et al. covering “Broadband Wire Grid Polarizer For the Visible Spectrum” and U.S. Pat. No. 6,665,119 of Kurtz et al. covering “Wire Grid Polarizer”.
U.S. Pat. No. 3,046,839 of Bird et al. covering “Process For Preparing Light Polarizing Materials” and U.S. Pat. No. 4,456,515 of Krueger et al. covering “Method For Making Polarizers Comprising a Multiplicity of Parallel Electrically Conductive Strips on a Glass Carrier” disclose photolithographic processes for forming wire grid polarizers that eliminate difficult etching steps. A thin layer of metal is deposited at an oblique angle to the substrate after a photolithographic pattern of finely spaced parallel lines is fabricated directly on a transparent substrate. The oblique angle of incidence, coupled with periodic topographic steps in the resist pattern, cause the metal to accumulate primarily on the sidewalls of the pattern. When photoresist is subsequently washed away, only the thin metal lines that are attached to the substrate between ridges of photoresist and accumulated on the sidewalls of the resist pattern remain.
Photolithographic techniques for reducing the periodicity of parallel conductive lines from approximately one micrometer (limiting the resultant devices to the near IR spectrum) to approximately 0.1 micrometer (suitable for the visible spectrum) has been disclosed, for example, by Karthe (see Wolfgang Karthe, “Nanofabrication Technologies and Device Integration”, Proceedings of SPIE, vol. 2213 (July 1994), pp. 288-296).
Techniques for fabricating wire grid polarizers by methods employing photolithography face inherent and well-recognized limitations. First, the lengths of the sides of the area that can be exposed during a single exposure (and, hence, the size of the polarizer) are limited to a few inches. This is far too small for most direct view displays such as those employed in laptop computers, television sets, cell phones, personal digital assistants (PDAs) and the like. Secondly, the cost of photolithographic processes is rather high due to the costs of high-resolution photolithography mask aligners, and the requisite ultra-high quality clean room facility required to house such a system.
Holographic photolithography has been used to form light and dark regions to expose photoresist. A very sophisticated optical setup and lasers are needed to do this, but one can expose photoresist with the interference patterns. However, the interference pattern that comes from interfering two oblique beams produces a periodicity that is not any smaller than the wavelength of the laser. Thus with visible lasers it's not possible to get to the 100-nm or smaller periodicity needed for visible polarizers. One would need an extreme ultra-violet wavelength laser and photoresists suitable for use in this spectral region are not commonly available.
Thus, there is a need in the art, for a wire grid polarizer large enough to be used for direct view displays and a method for fabricating such a wire grid polarizer.